VanSlyke, Tang and others have elucidated, e.g., in U.S. Pat. Nos. 4,539,507 and 4,720,432, and JP-A-05-234681 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") that the use of aromatic tertiary amines having phenyl, phenylene or biphenhylene groups in the hole injection-transport zone of an internal junction organic electroluminescence apparatus can elevate the light-output stability and lengthen the operational lifetime. For the purpose of further elevating the light-output stability, various modifications of aromatic tertiary amines used in the hole injection-transport zone have been tried by many investigators, and the results thereof have been applied for patents and published in academic journals. For instance, the modifications of biphenyl group-containing tertiary amines are disclosed, e.g., in Japanese Journal of Applied Physics, 27, L269 (1988), JP-A-59-194393, Appl. Phys. Lett., 66, 2679 (1995), JP-A-05-234681, JP-A-07-331238, JP-A-08-48656 and WO 95/09147; and those of starburst type tertiary amines are disclosed, e.g., in Appl. Phys. Lett., 65, 807 (1944) and JP-B-07-110940 (The term "JP-B" as used herein means an "examined Japanese patent publication").
Aromatic tertiary amines have been acquiring importance as an electrophotographic material also, and their utilization as a hole transporting material have been applied for patents. For instance, such amines are disclosed in JP-58-32372, JP-A-63-23594 and JP-A-01-142657. And effective use thereof is already made in midget plain-paper copying machines to contribute towards the popularization of low-priced copying machines.
As mentioned above, aromatic tertiary amines are of usefulness as an organic electroluminescent material and an electrophotographic material, but they are still inferior to inorganic materials in stabilities to heat and light. Therefore, improvements in their stabilities remain a weighty subject of research. More specifically, the problems confronting conventional aromatic tertiary amines as an organic electroluminescent material are as follows: Since the film of a conventional aromatic tertiary amine formed on an element by vacuum evaporation has insufficient stability, the properties thereof is changed by the crystallization occurring by lapse of time and being promoted by a rise in the element temperature due to the generation of heat during the operation to result in a lowering of the luminous efficiency, the appearance of a non-electroluminescent area called a dark spot and an increase in the number of dark spots, and further a rise in the voltage under constant-current operation. Thus, the element is ended up with a breakdown. This being the case, it has been desired to develop aromatic tertiary amines which can form stable films.